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Exceptions in the Contract

/**
   . . .
   @throws IllegalArgumentException if queue is empty
*/
public Message removeFirst() 
{ 
   if (count == 0) 
      throw new IllegalArgumentException();
   Message r = elements[head]; 
   . . .
} 

• Exception throw part of the contract
• Caller can rely on behavior
• Exception throw not result of precondition violation
• This method has no precondition
  

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